The invention relates to interferometers, and in particular to servo control systems and methods for controlling an optical path difference in a step scanning spectrometer interferometer.
A Fourier transform spectrometer typically includes an interferometer used to generate an interference between reflected portions of a monochromatic reference beam (e.g. a laser beam), and an interference between reflected portions of a broadband infrared beam of interest. The interferometer may include one or more movable mirrors, whose positions are used to control optical pathlengths through the interferometer. In rapid scanning, a pathlength difference (retardation) between the interfering beam portions is increased at a constant velocity over an interval of interest as data is acquired. In step scanning, the retardation is changed in steps, and data is acquired at each step. The retardation can be altered by moving one or more optical elements such as mirrors.
A step scanning interferometer can use dither to generate small cyclic changes in the path difference between a linearly-movable mirror and a mirror attached to an actuator such a piezoelectric transducer (PZT). The dither modulates both the monochromatic reference beam and the broadband infrared beam. The monochromatic light beam can be used as a reference to accurately servo control the average path difference between the two mirrors. In one approach, the modulated monochromatic beam from the interferometer is directed to a detector whose output is AC coupled. A demodulator operating at twice the dither frequency is used to detect a second harmonic of the dither signal and is applied as the error input of a servo that adjusts the time-averaged path difference between the moving mirror and fixed mirror to a zero crossing of the monochromatic light. The application of dither and AC coupling reduce the dependence of system properties on drift caused by time variations in monochromatic light source intensity, beam splitter efficiency, and detector sensitivity.
Commonly used interferometers may perform suboptimally in some IR spectroscopy applications, such as time-resolved spectroscopy (TRS).